(1) Field of the Invention
The invention concerns an apparatus and a method for determining the orientation of a crystallographic plane relative to a crystal surface as well as an apparatus and a method for cutting a single crystal in a cutting machine.
(2) Description of Related Art
For certain applications, semiconductor wafers with so-called misorientation are needed. As can be seen from FIG. 1, in a semiconductor wafer 1 with misorientation a certain crystallographic plane, e.g. the (100) plane, is not parallel to the wafer surface 2. The angle of misorientation φ is in this case the angle which the vector [100] which is perpendicular to the (100) plane forms with the normal vector N0 which is perpendicular to the wafer surface 2. If such misorientation is needed, then a single crystal from which the wafers are cut is tilted by the predefined angle φ about an axis T located in the cutting plane, i.e. the wafer surface 2.
With the known method of inner hole sawing, to produce such misorientation the orientation of the crystal which is stuck on a work piece holder is measured with an X-ray goniometer by measuring the position of the Bragg reflection relative to the work piece holder. By means of this holder, holding is effected on the inner hole cutting saw which has a horizontally and vertically displaceable support on which the measured orientation of the crystal can be corrected or adjusted to the desired value. The first wafer cut off is again measured on an X-ray goniometer, and the support is recorrected if necessary. Imprecisions in orientation which arise when the work piece holder is inserted in the inner hole sawing apparatus can thus be eliminated only by repeated measurement and recorrection.
With the known method of wire sawing, such correction by remeasuring and reorientation is not possible, because all the wafers are cut from a single crystal simultaneously. As can be seen from FIG. 2a, in wire sawing a single crystal 3 is held in a holder, not shown in FIG. 2a, which is movable by a drive of a feed unit towards the wire area 4 of a wire saw at a feed rate v and back to the starting position. The wire saw consists of a plurality of parallel wires 4a, 4b, 4c which are lightened by means of rollers, not shown in FIG. 2, and movable in planes perpendicular to the longitudinal centre axis M of the single crystal 3 in the directions shown by the arrows A and B in FIG. 2a. The wire sawing apparatus further includes devices 5 and 6 for applying a paste containing silicon carbide particles to the wires 4a, 4b, 4c on each side of the single crystal 3. In wire sawing with electrically bound cutting particles, a device for applying a cooling lubricant is further provided.
Wire saws are known with an orienting unit which for adjustment of the desired misorientation, as can be seen in FIG. 2b, allows exclusively displacement in a plane parallel to the plane of the wire area 4. For this purpose the crystal is measured outside the wire saw on an X-ray goniometer and stuck to a work piece substrate in such a way that the misorientation to be set lies in the horizontal plane, that is, the angle φ shown in FIG. 1 in a plane parallel to the wire area 4. The measurements of the X-ray goniometer in this case relate to a stop face of the work piece substrate, which is then applied to a reference face on the wire saw. Then the desired orientation is set horizontally. With this method, however, errors due to contamination of the stop and reference faces as well as adhesion errors which arise when the single crystal is stuck to the work piece substrate are not detected, as orientation measurement takes place outside the machine. Further, the single crystal must always be turned in such a way that the misorientation to be set lies in the horizontal plane parallel to the wire area 4. As a result, the direction of processing is governed by the required misorientation and can therefore vary from one single crystal to the next.
It is known from U.S. Pat. No. 5,904,136 that the required tilt angle for setting the misorientation can be performed in a tilting device outside a wire sawing apparatus, wherein the crystal orientation is determined with an X-ray device and then the crystal is tilted in the tilting device in horizontal and vertical directions relative to the wire area. Possible errors upon insertion of the tilting device together with the crystal in the wire sawing apparatus therefore likewise cannot be eliminated, however.
It is the object of the invention to provide an apparatus and a method for determining the orientation of a crystallographic plane relative to a crystal surface as well as an apparatus and a method for cutting a single crystal in a cutting machine, with which it is possible to carry out precise cutting and at the same time increase the yield of wafers during cutting of the single crystal.